The present invention relates to a method and apparatus for applying a coating such as a paint or a varnish on an article.
It is well known to cover the surface of an article of plastics material in a varnish that is designed, for example, to protect decoration provided thereon.
In conventional manner, the varnish is sprayed by means of a spray gun in a painting or varnishing station.
That method requires the use of paints or varnishes that need to be diluted in organic solvents, which solvents must be eliminated prior to polymerization.
Such elimination by evaporation is harmful to the environment.
In addition, solvent evaporation does not make it easy to recycle any paint or varnish that may be recovered.
Finally, the painting or varnishing station is generally situated at a location that is remote from the station where molding takes place, so while the article is being conveyed to the painting or varnishing station, it is exposed to being dirtied, and in some cases there is a risk of the decoration formed thereon being damaged.
There thus exists a need to reduce loss of varnish and to facilitate the operation of varnishing, and more generally to perform the operation of depositing a coating such as paint or varnish on an article in a manner that is more effective.
The present invention provides a novel method of applying a coating such as paint or varnish on a hollow article, in particular an article made of molded plastics material such as the body of a container, a container lid, a stopper, or a flask, with the opening of the flask possibly being narrower than the bottom thereof.
The method includes the operation consisting in depositing a predetermined quantity of coating in the fluid state on the center of a surface of the article, and in spreading it by causing the article to revolve.
By means of the invention, the coating is deposited on the article without needing prior dispersion in the atmosphere in the form of fine droplets.
The invention thus makes it possible to reduce the quantity of paint or varnish that comes into contact with the atmosphere, and thus to reduce losses of solvent by evaporation.
The invention thus makes it possible to use a conventional varnish or paint that does not have organic solvents and that is easy to recycle.
The varnish or paint without solvent is advantageously heated, preferably being raised to a temperature that is higher than ambient temperature, for example a temperature in the range 40xc2x0 C. to 50xc2x0 C., for the purpose of increasing its fluidity.
By way of example of a coating that does not have organic solvents, mention can be made of the following composition:
oligomers (urethane/acrylate oligomers);
polyfunctional monomers (acrylates) having strong double bonds; and
photoinitiators (3% to 8%, and preferably 4% to 5%).
The coating can be applied automatically in a closed enclosure, close to the molding station, since said enclosure is more compact than are traditional installations.
This reduces the extent to which the article is at risk during transport of possible dirtying and of damage to decoration on the article, if any.
In a particular implementation of the method of the invention, the surface on which the coating is deposited is situated inside the article, and the quantity of coating that is deposited is sufficient to enable it to rise under the effect of centrifugal force at least some of the way along the side wall of the article.
Surprisingly, the coating deposited on the bottom of the inside surface rises under the effect of centrifugal force along the side wall of the article, even if the side wall is vertical.
It is thus possible by means of the invention to paint an article whose side wall is parallel to the axis of rotation, whether or not the wall is a body of revolution.
Thus, in an implementation of the method of the invention, the side wall of the article is at least in part cylindrical about the same axis as the axis of rotation about which the article is driven.
The side wall of the article can be stepped.
Still in a particular implementation of the method of the invention, a quantity of coating is deposited on the bottom of the article that is sufficient to enable it to cover the top edge of the article after it has moved up along the side wall thereof.
The invention also makes it possible to paint or varnish articles which cannot be painted or varnished by conventional spraying, either because the opening is too small to enable a spray nozzle to be inserted inside the article, or else because the air present in the bottom of the article constitutes a buffer of gas that tends to oppose diffusion of the sprayed aerosol.
The invention thus makes it possible to paint an article of small section or a flask whose opening is narrower than its bottom.
When the coating that is used does not include any solvent, there is no harm in depositing the coating in excess on the surface to be coated since it is easy to recycle any residual quantity of coating that is ejected from the article during rotation thereof without any risk of harming the environment.
After the coating has spread over the inside surface of the article, rotary drive to the article is preferably stopped immediately, e.g. by stopping the drive mechanism, for the purpose of avoiding any coating running over the outside surface of the article.
In a particular implementation of the method of the invention, the coating which spreads under the effect of centrifugal force is deposited on a surface which is itself substantially plane or slightly convex and situated on the outside of the article.
To paint or varnish such a surface, the method advantageously includes the operations consisting in:
depositing a predetermined quantity of coating in the fluid state on the center of said surface, and spreading it by causing the article to spin; and
applying the coating directly on at least a portion of the remainder of the surface of the article.
These two operations can begin simultaneously or they can be performed one after the other, in whichever order is most suitable for the shape of the article that is to be painted or varnished.
The coating can be applied by means of a jet or a spray or by any other means, and it can be applied over the entire outside surface of the article that is not covered by the coating spreading under the effect of centrifugal force.
Advantageously, while the coating for spreading under the effect of centrifugal force is being deposited on the outside surface of the article, its speed of rotation is zero or very small (e.g. less than 5 revolutions per second (rps)) to avoid any risk of coating being thrown off on first contact with the article.
Preferably, on starting up, the speed of rotation of the article increases very quickly so as to avoid any running associated with the shape of the surface over which the coating spreads.
For example, the article can reach a speed of rotation lying in the range 50 rps to 100 rps in less than 1 second.
The coating can be applied directly to the outside surface of the article by means of a nozzle whose position and orientation are adjustable.
Thus, in a particular implementation of the method of the invention, where the substantially plane or slightly convex surface is generally rectangular in shape when observed from above at the moment when it is set into rotation for spreading the coating under the effect of centrifugal force, the means for applying the coating directly to the surface of the article comprise a nozzle that slopes downwards and that is situated slightly above the periphery of said substantially plane or slightly convex surface.
Advantageously, to paint or varnish inside and/or outside surfaces of an article, the coating used is one that is capable of being cured under the effect of ultraviolet radiation.
Curing of the coating deposited on the article is preferably induced while the article is still rotating.
The invention also provides apparatus for applying a coating on an article having at least one surface that is substantially plane or slightly convex, the apparatus comprising:
a rotary support for rotating the article;
a dispenser member for depositing a predetermined quantity of coating in the fluid state on a predetermined location of said surface; and
means for applying the coating directly on at least a portion of the surface of the article that is not covered by the coating being spread under the effect of centrifugal force.
The term xe2x80x9ca surface which is substantially plane or slightly convexxe2x80x9d is used to designate any surface which can be covered by spreading under the effect of centrifugal force spreading a coating deposited in the center thereof.
The apparatus preferably has an enclosure within which the article is rotated.
Also preferably, the means for applying the coating directly on the article comprise a nozzle whose inclination and positioning relative to the rotary support are adjustable.
When the paint or varnish used can be cured under the action of ultraviolet radiation, the apparatus further includes means for exposing the article to ultraviolet radiation.
In a particular embodiment, the dispenser member is connected to a tank containing a coating without organic solvent.
Still in a particular embodiment, the apparatus includes means for heating the coating.
The invention is particularly suitable for applying a coating to a hollow article such as the body of a container, a container lid, a stopper, or a flask.
When the article has an outside surface that is substantially plane or slightly convex that is generally rectangular in shape when observed from above at the time when it is rotated to spread the coating under the effect of centrifugal force, the means for applying the coating directly to the remainder of the surface of the article advantageously include a downwardly-sloping nozzle situated slightly above the periphery of said substantially plane or slightly convex surface.
It has been observed, quite surprisingly, that the invention makes it possible to paint or varnish in satisfactory manner articles that are not circularly symmetrical, and in particular lids of rectangular shape.
It has been found that by applying the paint or varnish in the form of a jet or a spray to the sides of the lid, it is possible when the nozzle is suitably placed to cover the corners of the lid which are not reached by the paint or varnish spreading under the effect of centrifugal force, but without that giving rise to excess paint or varnish at any location on the surface of the article.
The invention also provides a painted or varnished hollow article having a surface covered in a coating spread under the effect of centrifugal force.
The article may have at least a portion of its surface which is covered by a coating that is deposited by direct application, e.g. by being delivered by means of a nozzle.
The thickness of the deposited coating lies advantageously in the range 3 xcexcm to 30 xcexcm, and preferably in the range 8 xcexcm to 25 xcexcm.
The article may constitute the body of a container, a container lid, a stopper, or a flask.
The coating used is advantageously a paint or a varnish without organic solvent.